360 OF RESPIRATION. 



eight times more blood from the veins of the penis at the period of erection 

 than during the intervals. The ischio-cavernosi and bulbo cavernosus mus- 

 cles probably afford some assistance in completing and sti*engtheuing the act 

 of erection by compressing the veins which return the blood from the penis, 

 but they are clearly unable to effect it by their own act, and it is obvious 

 that no analogous power can be exerted in other erectile organs as the 

 nipple. Their centres, according to Goltz, 1 are situated in the lumbar region 

 of the spinal cord, and here also is, in all probability, an inhibitory centre 

 governing their action as well as that of the sphincter ani. 



CHAPTER IX. 



OF RESPIRATION. 



1. Nature of the Function; and Provisions for its Performance. 



279. THE Nutritive fluid, in its circulation through the capillaries of the 

 system, undergoes great alterations, both in it physical constitution and in 

 its vital properties. It gives up to the tissues with which it is brought into 

 contact, some of its most important elements; and at the same time it is 

 made the vehicle of the removal, from these tissues, of ingredients which 

 are no longer in the state of combination that fits them for their offices in 

 the Animal Economy. To separate these ingredients from the general cur- 

 rent of the circulation, and to carry them out of the system, is the great 

 object of the excretory organs; the importance of whose respective func- 

 tions will vary, it is very evident, with the amount of the ingredient which 

 they have to separate, and with the deleterious influence which its retention 

 would exert on the welfare of the system at large. Of all these injurious 

 ingredients, Carbonic Acid is without doubt the one most abundantly intro- 

 duced into the nutritive fluid ; and it is also most deleterious in its effects on 

 the system, if allowed to accumulate. We find, accordingly, that the pro- 

 vision for the removal of this substance from the blood is one of peculiar 

 extent and importance, especially in the higher forms of animals; and fur- 

 ther, that instead of being effected by an operation peculiarly vital (like 

 other acts of Excretion ), its performance is secured by being made to depend 

 upon simple j>////.si'ca/ conditions, and is thus comparatively little susceptible 

 of derangement from disorder of other processes. All that is requisite for 

 it, is the exposure of the Blood to the influence of the Atmospheric air (or, 

 in aquatic animals, of air dissolved in water), 7 through the medium of a 

 membrane that shall permit the "diffusion of gases;" an interchange then 

 taking place between the gaseous matters on the two sides, Carbonic Acid 

 being exhaled from the Blood, and being replaced by oxygen from the air. 

 Thus the extrication of Carbonic acid is eil'ected in a manner that renders 

 it subservient to the introduction of that element which is required for all 



1 (ioltx, rilii-.-r's Arcliiv, Hand vii, 1873, p. , r >82. 



- Whil>t nir contain- about, !-tli of its volume of oxygen pis, water contains only 

 .},,,tli of its volume The feebleness of the; respiratory processes which this implies 

 in aquatic animal* is in part compensated for by the ready solubility of carbonic acid 

 r:is in water, which facilitates its e.-capc from the s\>tcm, and the circumstance that 

 the respiratory organs are actually floating in the oxygenated medium. 



